Various processes have been developed for casting metals using a destructable pattern made of material that vaporizes on contact with molten metal when poured into a mold containing the destructable pattern. Such processes, typically referred to as "Evaporative Pattern Casting" processes, commonly use expandable polystyrene beads which are formed into the desired foundry patterns. Molten metal, such as iron or steel poured into a mold containing sand surrounding the polystyrene pattern, causes the pattern to vaporize, resulting in a metal replica of the pattern in the sand. Upon decomposition of the polystyrene, however, a significant amount of carbonaceous residue is formed on the surface of the metal casting. This residue, thought to be pyrolytic carbon, interferes with the quality of the metal casting making this process unattractive for operations where high quality castings are desired.
Attempts have been made to replace the polystyrene foam with foams made from phenolic-urethane and urea formaldehyde polymers. These compounds require very high temperatures for decomposition and, like the polystyrene, produce carbonaceous residues upon decomposition. An extensive ventilation system is also required when using these type of foams because of the large quantities of toxic gases generated upon vaporization.
U.S. Pat. No. 3,351,123 discloses an evaporative pattern casting process and a mold for use in such a process. The mold comprises a foamed thermoplastic synthetic resin having a refractory coating consisting of a particulate refractory filler bonded together with a silicon-containing binding agent. The thermoplastic synthetic resins which are disclosed for use in this operation include: cellulose ethers and esters, olefins, vinyl esters of carboxylic acids, vinyl ethers, unsaturated carboxylic acids and derivatives thereof, methacrylic acid esters of alcohols containing 1-18 carbon atoms, vinyl aromatic compounds such as styrene, and polymers of monomeric compounds containing the vinylidene grouping CH.sub.2 .dbd.C&lt;.
U.S. Pat. No. 3,942,583 discloses using plastic patterns for metal casting molds which are built up from plastic plate or sheet material bonded to an internal supporting frame. The plastic material is comprised of foam plastic, such as foams of polyurethane, phenol resin, polystyrene, cellulose acetate, polyvinylchloride, polyethylene or vinylacetate. Alternatively, the material can also be a solid plastic such as polyethylene.